1. Technical Field of the Invention
The present invention relates generally to a drive circuit in which a high-side switching circuit and a low-side switching circuit are connected in series between power supply lines through an output terminal leading to a switching element to be controlled by the drive circuit, and more particularly to a reduced current and power consumption structure of such a drive circuit.
2. Background Art
FIGS. 9 and 10 show a conventional drive circuit 1 designed to control an on-off operation of a switching element.
The drive circuit 1 is fabricated on an IC and designed to receive a control signal Sa inputted to an input terminal 2 to provide a drive voltage signal Vo to, for example, a gate of an n-channel MOSFET 4 coupled to an output terminal 3. The drive circuit 1 includes a high-side transistor T1, a low-side transistor T2, and a resistor R1. The high-side transistor T1 and the low-side transistor T2 are connected at collectors and emitters thereof in series between a positive power supply line 5 and a negative power supply line 6 (also referred to as a ground line 6 below) through the resistor R1. A junction of the resistor R1 and the collector of the low-side transistor T2 is coupled to the output terminal 3.
The drive circuit 1 also includes predrivers 7 and 8, a transistor T3, and a constant current source CS1. The transistor T3 is, as clearly shown in FIG. 10, disposed between the input terminal 2 and input terminals of the predrivers 7 and 8. The constant current source CS1 works to provide a bias current to the transistor T3. The predrivers 7 and 8 receive a signal from the transistor T3 to operate in logical forms reverse to each other and drive the transistors T1 and T2, respectively.
Specifically, the drive circuit 1 is a push-pull circuit which is responsive to the control signal Sa of a low level inputted to the input terminal 2 to turn on the transistor T1 and off the transistors T2 and T3 so that the voltage Vb is applied from the power supply line 5 to the gate of the MOSFET 4 to turn on the MOSFET 4. When the control signal Sa of a high level is inputted to the input terminal 2, the transistors T2 and T3 are turned on, while the transistor T1 is turned off, so that the voltage of zero (0) at the ground line 6 is applied to the gate of the MOSFET 4 to turn off the MOSFET 4.
FIG. 9 illustrates the structure of the drive circuit 1. The predrivers 7 and 8 have common elements for simplifying structures thereof. Specifically, the predrivers 7 and 8 are made up of the transistor T4, a transistor T5, a transistor T6, and resistors R2 to R8. The transistor T4 works to turn off the transistor T1 and turns on the transistor T2. The transistor T5 works to turn off the transistor T2. The transistor T6 works to drive the transistor T4.
When the transistor T3 is turned off by the control signal Sa of the low level, it will cause the transistors T5 and T6 to be turned on and the transistor T4 to be turned off, so that the transistor T1 is turned on, and the transistor T2 is turned off. Alternatively, when the transistor T3 is turned on by the control signal Sa of the high level, it will cause the transistors T5 and T6 to be turned off and the transistor T4 to be turned on, so that the transistor T1 is turned off, and the transistor T2 is turned on.
Between the gate and the source and between the gate and the drain of the MOSFET 4, capacitors Cgs and Cgd are usually provided, respectively. These gate capacitors are illustrated by broken lines in FIG. 10. Decreasing turning-on and -off time periods of the MOSFET 4 to achieve a rapid switching operation thereof requires an increased ability of the drive circuit 1 to produce a great current for charging and discharging the gate capacitors of the MOSFET 4 when required to be switched between the on-state and the off-state.
Accordingly, in the drive circuit 1, the base current of the transistor T2 is set to a great value for enabling the transistor T2 to withdraw as the collector current thereof the electric charge from the gate capacitors for a short time when the MOSFET 4 is switched from the on-state to the off-state. Additionally, the base current of the transistor T1 is set to a great value for enabling the transistor T1 to charge the gate capacitors of the MOSFET 4 for a short time with the collector current thereof when the MOSFET 4 is switched from the off-state to the on-state. The adjustment of these base currents is achieved by regulating the resistance values of the resistors R4 and R6.
The base current of the transistor T2 inputted from the power supply line 5 through the resistor R6 and the transistor T4 continues to flow not only when the transistor T2 is switched between the on- and off-states, but also during a steady-state operation in which the transistor T2 is in the on-state (i.e., the MOSFET 4 is in the off-state). An increase in base current of the transistor T2 for shortening the turning-off time period thereof, thus, causes the current consumption of the drive circuit 1 to increase, which results in an increase in quantity of heat generated by the resistors R4 and R6. This requires a decrease in guarantee ambient temperature of the IC in which the drive circuit 1 is located.
In a case of a drive circuit (not shown) designed to drive a p-channel MOSFET, a great base current of a high-side transistor continues to flow when it is in the on-state for the same reasons as described above, thus resulting in an increase in energy loss of the drive circuit.